Key Diagram of 66 kV / 11 kV Substation

Key Diagram of 66 kV / 11 kV Substation

The key diagram of 66 kV/11 kV substation is as shown in the Fig. 8.7.1. The explanation of the key diagram is given below.

Typically such a substation has 2 incoming 66 kV lines which can be labelled as 'incoming 1' and 'incoming 2.' These lines are connected to busbars. The arrangement of two incoming lines shown in Fig. 8.7.1 forms double circuit or double bus bar system having two bus bars one called "main" bus bar and other called "spare" bus bar.

These two bus bars can take entire substation load independently or share the load when put in service simultaneously. Under the event of breakdown of one of the incoming lines, the continuity of the supply can be maintained by other line. Thus this arrangement increases reliability of the system.

The incoming lines are connected to either of the bus bars, namely main or spare, with the help of a bus coupler consisting of circuit breaker and isolators. If repair work is to be carried out on one of the bus bars, the supply is not required to be interrupted and the total load is transfered to the other bus.

The same 66 kV double circuit supply is going out i.e. 66 kV double circuit supply is passing through the substation. The outgoing 66 kV double circuit line can be made to act as incoming line.

There is also an arrangement to step down the voltage from 66 kV to 11 kV with the help of two units of three phase transformers. Each of the unit of the transformer is supplying to a separate bus bar. One of the transformers can be loaded with entire substation load while other transformer acts as spare unit. Both transformers can be operated simultaneously to share the total load depending upon the requirement. The 11 kV outgoing lines going out from these substations are fed to the distribution substations located near the consumers.

The incoming and outgoing lines are connected through circuit breakers with isolators on either of its side. Under the repair work over the line tower, the line is first switched off by operating circuit breakers and isolators. The line is then earthed to remove the trapped charge from it.

The instrument transformers such as CTs and PTs are installed for metering and protection purposes. The output of CTs and PTs are given to metering and indicating instruments and relay circuits. At the point of line termination, PTs are installed while CTs are placed at the terminals of each circuit breaker.

The lightning arresters are connected near the transformers on H.T. side to protect the system from lightning surges.

The auxiliary components such as capacitor bank, earth connections, local supply connections, d.c. supply connections. The capacitor bank is used for power factor improvement. These components are not shown in key diagram for simplicity.

Review Question

1. Draw and explain key diagram of 66 kV/11 kV substation.